An optical communication system can transmit and receive large-volume information at high speed by transmitting signal light through an optical fiber transmission line. In the optical communication system, there is a case where signal light of L-band (1565 nm to 1625 nm) is used, as well as a case where signal light of C-band (1530 nm to 1565 nm) is used. Also, the optical communication system is equipped with optical fiber amplifiers arranged along the signal light transmission path so that signal light can be transmitted over a long distance by optically amplifying the signal light.
The optical fiber amplifier is equipped with an optical fiber for amplification and a light source for outputting pump light to be supplied to the optical fiber, where the optical fiber is basically made of silica glass in which the core region is doped with a rare-earth element. In the optical fiber amplifier, the rare-earth element added to the core region of the optical fiber for amplification is excited by the pump light, and accordingly signal light is optically amplified in the optical fiber for amplification.
Optical fiber amplifiers or optical fibers for amplification for optically amplifying L-band signal light are disclosed in Japanese Patent No. 3690110, Japanese translation of PCT Application Publication No. 2004-513056, Japanese Patent Application Publication No. 2003-124547, and Japanese Patent Application Publication No. 2003-086870. In these L-band optical fibers for amplification, their core regions are doped with P-elements in addition to rare-earth elements. Also, in these optical fibers for amplification, Ge elements are added as a refractive index raising agent to the core regions in order to increase the refractive index of the core regions higher than the refractive index of the cladding regions.
In Japanese Patent No. 3690110, it is stated that preferably the concentration of P-element in the optical fiber for amplification is 5 wt % or more, or 8 wt % or more. In Japanese translation of PCT Application Publication No. 2004-513056, it is stated that the P2O5 concentration in the optical fiber for amplification is equal to or more than 5 wt % and preferably 8 wt % or more. According to Japanese Patent Application Publication No. 2003-124547, the P-element concentration of the optical fiber for amplification is from 1 wt % to 10 wt %. Also, according to Japanese Patent Application Publication No. 2003-086870, the preferable P-element concentration of the optical fiber for amplification is 0.01 mol % or more and 10 mol % or less.
However, according to the finding of the present inventor, these L-band optical fibers for amplification have the following problems. That is, when the concentration of P element is high, the structural defect in the silica glass increases, and consequently the transmission loss increases, resulting in degradation of long-range reliability (particularly hydrogen-resistant characteristic). Furthermore, if Ge element is added, in addition to the P element, to the core region, the transmission loss increases due to the added Ge element, and the hydrogen-resistant characteristic is degraded.    [Patent document 1] Japanese Patent No. 3690110    [Patent document 2] Japanese translation of PCT Application Publication No. 2004-513056    [Patent document 3] Japanese Patent Application Publication No. 2003-124547    [Patent document 4] Japanese Patent Application Publication No. 2003-086870